Mode-locked multi-mode fiber laser pulse source

ABSTRACT

A laser utilizes a cavity design which allows the stable generation of high peak power pulses from mode-locked multi-mode fiber lasers, greatly extending the peak power limits of conventional mode-locked single-mode fiber lasers. Mode-locking may be induced by insertion of a saturable absorber into the cavity and by inserting one or more mode-filters to ensure the oscillation of the fundamental mode in the multi-mode fiber. The probability of damage of the absorber may be minimized by the insertion of an additional semiconductor optical power limiter into the cavity.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/785,944 filed Feb. 16, 2001, which is a continuation of U.S. patentapplication Ser. No. 09/199,728 filed Nov. 25, 1998, now U.S. Pat. No.6,275,512 issued Aug. 14, 2001.

FIELD OF THE INVENTION

The present invention relates to the amplification of single mode lightpulses in multi-mode fiber amplifiers, and more particularly to the useof multi-mode amplifying fibers to increase peak pulse power in amode-locked laser pulse source used for generating ultra-short opticalpulses.

BACKGROUND OF THE INVENTION Background Relating to Optical Amplifiers

Single-mode rare-earth-doped optical fiber amplifiers have been widelyused for over a decade to provide diffraction-limited opticalamplification of optical pulses. Because single mode fiber amplifiersgenerate very low noise levels, do not induce modal dispersion, and arecompatible with single mode fiber optic transmission lines, they havebeen used almost exclusively in telecommunication applications.

The amplification of high peak-power pulses in a diffraction-limitedoptical beam in single-mode optical fiber amplifiers is generallylimited by the small fiber core size that needs to be employed to ensuresingle-mode operation of the fiber. In general the onset ofnonlinearities such as self-phase modulation lead to severe pulsedistortions once the integral of the power level present inside thefiber with the propagation length exceeds a certain limiting value. Fora constant peak power P inside the fiber, the tolerable amount ofself-phase modulation Φ_(n1) is given by

${\Phi_{nl} = {\frac{2\pi\; n_{2}{PL}}{\lambda\; A} \leq 5}},$where A is the area of the fundamental mode in the fiber, ë is theoperation wavelength, L is the fiber length and n₂=3.2×10⁻²⁰ m²/W is thenonlinear refractive index in silica optical fibers.

As an alternative to single-mode amplifiers, amplification in multi-modeoptical fibers has been considered. However, in general, amplificationexperiments in multi-mode optical fibers have led tonon-diffraction-limited outputs as well as unacceptable pulse broadeningdue to modal dispersion, since the launch conditions into the multi-modeoptical fiber and mode-coupling in the multi-mode fiber have not beencontrolled.

Amplified spontaneous emission in a multi-mode fiber has been reduced byselectively exciting active ions close to the center of the fiber coreor by confining the active ions to the center of the fiber core. U.S.Pat. No. 5,187,759, hereby incorporated herein by reference. Since theoverlap of the low-order modes in a multi-mode optical fiber is highestwith the active ions close to the center of the fiber core, anyamplified spontaneous emission will then also be predominantly generatedin low-order modes of the multi-mode fiber. As a result, the totalamount of amplified spontaneous emission can be reduced in themulti-mode fiber, since no amplified spontaneous emission is generatedin high-order modes.

As an alternative for obtaining high-power pulses, chirped pulseamplification with chirped fiber Bragg gratings has been employed. Oneof the limitations of this technique is the relative complexity of theset-up.

More recently, the amplification of pulses to peak powers higher than 10KW has been achieved in multi-mode fiber amplifiers. See U.S. Pat. No.5,818,630, entitled Single-Mode Amplifiers and Compressors Based onMulti-Mode Fibers, assigned to the assignee of the present invention,and hereby incorporated herein by reference. As described therein, thepeak power limit inherent in single-mode optical fiber amplifiers isavoided by employing the increased area occupied by the fundamental modewithin multi-mode fibers. This increased area permits an increase in theenergy storage potential of the optical fiber amplifier, allowing higherpulse energies before the onset of undesirable nonlinearities and gainsaturation. To accomplish this, that application describes theadvantages of concentration of the gain medium in the center of themulti-mode fiber so that the fundamental mode is preferentiallyamplified. This gain-confinement is utilized to stabilize thefundamental mode in a fiber with a large cross section by gain guiding.

Additionally, that reference describes the writing of chirped fiberBragg gratings onto multi-mode fibers with reduced mode-coupling toincrease the power limits for linear pulse compression of high-poweroptical pulses. In that system, double-clad multi-mode fiber amplifiersare pumped with relatively large-area high-power semiconductor lasers.Further, the fundamental mode in the multi-mode fibers is excited byemploying efficient mode-filters. By further using multi-mode fiberswith low mode-coupling, the propagation of the fundamental mode inmulti-mode amplifiers over lengths of several meters can be ensured,allowing the amplification of high-power optical pulses in dopedmulti-mode fiber amplifiers with core diameters of several tens ofmicrons, while still providing a diffraction limited output beam. Thatsystem additionally employed cladding pumping by broad area diode arraylasers to conveniently excite multi-mode fiber amplifiers.

BACKGROUND RELATING TO MODE-LOCKED LASERS

Both actively mode-locked lasers and passively mode-locked lasers arewell known in the laser art. For example, compact mode-locked lasershave been formed as ultra-short pulse sources using single-moderare-earth-doped fibers. One particularly useful fiber pulse source isbased on Kerr-type passive mode-locking. Such pulse sources have beenassembled using widely available standard fiber components to providepulses at the bandwidth limit of rare-earth fiber lasers with GigaHertzrepetition rates.

Semiconductor saturable absorbers have recently found applications inthe field of passively mode-locked, ultrashort pulse lasers. Thesedevices are attractive since they are compact, inexpensive, and can betailored to a wide range of laser wavelengths and pulsewidths. Quantumwell and bulk semiconductor saturable absorbers have also been used tomode-lock color center lasers

A saturable absorber has an intensity-dependent loss l. The single passloss of a signal of intensity I through a saturable absorber ofthickness d may be expressed asl=1−exp(−αd)in which α is the intensity dependent absorption coefficient given by:α(I)=α₀/(1+I/I _(SAT))Here α₀ is the small signal absorption coefficient, which depends uponthe material in question. I_(SAT) is the saturation intensity, which isinversely proportional to the lifetime (τ_(A)) of the absorbing specieswithin the saturable absorber. Thus, saturable absorbers exhibit lessloss at higher intensity.

Because the loss of a saturable absorber is intensity dependent, thepulse width of the laser pulses is shortened as they pass through thesaturable absorber. How rapidly the pulse width of the laser pulses isshortened is proportional to |dq₀/dI|, in which q₀ is the nonlinearloss:q ₀ =l(I)−l(I=0)l(I=0) is a constant (=1−exp(−α₀d)) and is known as the insertion loss.As defined herein, the nonlinear loss q₀ of a saturable absorberdecreases (becomes more negative) with increasing intensity I. |dq₀/dI|stays essentially constant until I approaches I_(SAT), becomingessentially zero in the bleaching regime, i.e., when I>>I_(SAT).

For a saturable absorber to function satisfactorily as a mode-lockingelement, it should have a lifetime (i.e., the lifetime of the upperstate of the absorbing species), insertion loss l(I=0), and nonlinearloss q₀ appropriate to the laser. Ideally, the insertion loss should below to enhance the laser's efficiency, whereas the lifetime and thenonlinear loss q₀ should permit self-starting and stable cwmode-locking. The saturable absorber's characteristics, as well as lasercavity parameters such as output coupling fraction, residual loss, andlifetime of the gain medium, all play a role in the evolution of a laserfrom startup to mode-locking.

As with single-mode fiber amplifiers, the peak-power of pulses frommode-locked single-mode lasers has been limited by the small fiber coresize that has been employed to ensure single-mode operation of thefiber. In addition, in mode-locked single-mode fiber lasers, theround-trip nonlinear phase delay also needs to be limited to around

to prevent the generation of pulses with a very large temporallyextended background, generally referred to as a pedestal. For a standardmode-locked single-mode erbium fiber laser operating at 1.55 μm with acore diameter of 10 μm and a round-trip cavity length of 2 m,corresponding to a pulse repetition rate of 50 MHz, the maximumoscillating peak power is thus about 1 KW.

The long-term operation of mode-locked single-mode fiber lasers isconveniently ensured by employing an environmentally stable cavity asdescribed in U.S. Pat. No. 5,689,519, entitled Environmentally StablePassively Mode-locked Fiber Laser Pulse Source, assigned to the assigneeof the present invention, and hereby incorporated herein by reference.The laser described in this reference minimizes environmentally inducedfluctuations in the polarization state at the output of the single-modefiber. In the described embodiments, this is accomplished by including apair of Faraday rotators at opposite ends of the laser cavity tocompensate for linear phase drifts between the polarization eigenmodesof the fiber.

Recently the reliability of high-power single-mode fiber laserspassively mode-locked by saturable absorbers has been greatly improvedby implementing non-linear power limiters by insertion of appropriatesemiconductor two-photon absorbers into the cavity, which minimizes thepeak power of the damaging Q-switched pulses often observed in thestart-up of mode-locking and in the presence of misalignments of thecavity. See U.S. patent application Ser. No. 09/149,369, filed on Sep.8, 1998, entitled Resonant Fabry-Perot Semiconductor Saturable Absorbersand Two-Photon Absorption Power Limiters, assigned to the assignee ofthe present invention, and hereby incorporated herein by reference.

To increase the pulse energy available from mode-locked single-modefiber lasers the oscillation of chirped pulses inside the laser cavityhas been employed. M. Hofer et al., Opt. Lett., vol. 17, page 807-809.As a consequence the pulses are temporally extended, giving rise to asignificant peak power reduction inside the fiber laser. However, thepulses can be temporally compressed down to approximately the bandwidthlimit outside the laser cavity. Due to the resulting high peak power,bulk-optic dispersive delay lines have to be used for pulse compression.For neodymium fiber lasers, pulse widths of the order of 100 fs can beobtained.

The pulse energy from mode-locked single-mode fiber lasers has also beenincreased by employing chirped fiber gratings. The chirped fibergratings have a large amount of negative dispersion, broadening thepulses inside the cavity dispersively, which therefore reduces theirpeak power and also leads to the oscillation of high-energy pulsesinside the single-mode fiber lasers.

See U.S. Pat. No. 5,450,427, entitled Technique for the Generation ofOptical Pulses in Mode-Locked Lasers by Dispersive Control of theOscillation Pulse Width, and U.S. Pat. No. 5,627,848, entitled Apparatusfor Producing Femtosecond and Picosecond Pulses from Fiber LasersCladding Pumped with Broad Area Diode Laser Arrays, both of which areassigned to the assignee of the present invention and herebyincorporated herein by reference. In these systems, the generated pulsesare bandwidth-limited, though the typical oscillating pulse widths areof the order of a few ps.

However, though the dispersive broadening of the pulse width oscillatinginside a single-mode fiber laser cavity does increase the oscillatingpulse energy compared to a ‘standard’ soliton fiber laser, it does notincrease the oscillating peak power. The maximum peak power generatedwith these systems directly from the fiber laser is still limited toaround I KW.

Another highly integratable method for increasing the peak power ofmode-locked lasers is based on using chirped periodically poled LiNb0₃(chirped PPLN). Chirped PPLN permits simultaneous pulse compression andfrequency doubling of an optically chirped pulse. See U.S. patentapplication Ser. No. 08/845,410, filed on Apr. 25, 1997, entitled Use ofAperiodic Quasi-Phase-Matched Gratings in Ultrashort Pulse Sources,assigned to the assignee of the present application, and herebyincorporated herein by reference. However, for chirped PPLN to producepulse compression from around 3 ps to 300 fs and frequency doubling withhigh conversion efficiencies, generally peak powers of the order ofseveral KW are required. Such high peak powers are typically outside therange of mode-locked single-mode erbium fiber lasers.

Broad area diode laser arrays have been used for pumping of mode-lockedsingle-mode fiber lasers, where very compact cavity designs werepossible. The pump light was injected through a V-groove from the sideof double-clad fiber, a technique typically referred to as side-pumping.However, such oscillator designs have also suffered from peak powerlimitations due to the single-mode structure of the oscillator fiber.

It has also been suggested that a near diffraction-limited output beamcan be obtained from a multi-mode fiber laser when keeping the fiberlength shorter than 15 mm and selectively providing a maximum amount offeedback for the fundamental mode of the optical fiber. “Efficient laseroperation with nearly diffraction-limited output from a diode-pumpedheavily Nd-doped multi-mode fiber”, Optics Letters, Vol. 21, pp. 266-268(1996) hereby incorporated herein by reference. In this technique,however, severe mode-coupling has been a problem, as the employedmulti-mode fibers typically support thousands of modes. Also, only anair-gap between the endface of the multi-mode fiber and a laser mirrorhas been suggested for mode-selection. Hence, only very poor modaldiscrimination has been obtained, resulting in poor beam quality.

While the operation of optical amplifiers, especially in the presence oflarge seed signals, is not very sensitive to the presence of spuriousreflections, the stability of mode-locked lasers critically depends onthe minimization of spurious reflections. Any stray reflections producesub-cavities inside an oscillator and result in injection signals forthe cw operation of a laser cavity and thus prevent the onset ofmode-locking. For solid-state Fabry-Perot cavities a suppression ofintra-cavity reflections to a level <<1% (in intensity) is generallybelieved to be required to enable the onset of mode-locking.

The intra-cavity reflections that are of concern in standard mode-lockedlasers can be thought of as being conceptually equivalent tomode-coupling in multi-mode fibers. Any mode-coupling in multi-modefibers clearly also produces a sub-cavity with a cw injection signalproportional to the amount of mode-coupling. However, the suppression ofmode-coupling to a level of <<1% at any multi-mode fiber discontinuitiesis very difficult to achieve. Due to optical aberrations, evenwell-corrected optics typically allow the excitation of the fundamentalmode in multi-mode fibers only with maximum efficiency of about 95%.Therefore to date, it has been considered that mode-locking of amulti-mode fiber is impossible and no stable operation of a mode-lockedmulti-mode fiber laser has yet been demonstrated.

SUMMARY OF THE INVENTION

This invention overcomes the foregoing difficulties associated with peakpower limitations in mode-locked lasers, and provides a mode-lockedmulti-mode fiber laser.

This laser utilizes cavity designs which allow the stable generation ofhigh peak power pulses from mode-locked multi-mode fiber lasers, greatlyextending the peak power limits of conventional mode-locked single-modefiber lasers. Mode-locking may be induced by insertion of a saturableabsorber into the cavity and by inserting one or more mode-filters toensure the oscillation of the fundamental mode in the multi-mode fiber.The probability of damage of the absorber may be minimized by theinsertion of an additional semiconductor optical power limiter into thecavity. The shortest pulses may also be generated by taking advantage ofnonlinear polarization evolution inside the fiber. The long-termstability of the cavity configuration is ensured by employing anenvironmentally stable cavity. Pump light from a broad-area diode lasermay be delivered into the multi-mode fiber by employing acladding-pumping technique.

With this invention, a mode-locked fiber laser may be constructed toobtain, for example, 360 fsec near-bandwidth-limited pulses with anaverage power of 300 mW at a repetition rate of 66.7 MHz. The peak powerof these exemplary pulses is estimated to be about 6 KW.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description of the preferred embodiments of the inventionreferences the appended drawings, in which like elements bear identicalreference numbers throughout.

FIG. 1 is a schematic illustration showing the construction of apreferred embodiment of the present invention which utilizes end-pumpingfor injecting pump light into the multi-mode fiber.

FIG. 2 is a graph showing the typical autocorrelation of pulsesgenerated by the invention of FIG. 1.

FIG. 3 is a graph showing the typical pulse spectrum generated by theinvention of FIG. 1.

FIG. 4 is a schematic illustration showing the construction of analternate preferred embodiment utilizing a side-pumping mechanism forinjecting pump light into the multi-mode fiber.

FIG. 5 is a schematic illustration of an alternative embodiment whichuses a length of positive dispersion fiber to introduce chirped pulsesinto the cavity.

FIG. 6 is a schematic illustration of an alternative embodiment whichuses chirped fiber gratings with negative dispersion in the laser cavityto produce high-energy, near bandwidth-limited pulses.

FIGS. 7A and 7B illustrate polarization-maintaining multi-mode fibercross sections which may be used to construct environmentally stablecavities in the absence of Faraday rotators.

FIG. 8 is a schematic illustration of an alternative embodiment whichutilizes one of the fibers illustrated in FIGS. 7A and 7B.

FIGS. 9A, 9B and 9C illustrate the manner in which the fundamental modeof the multi-mode fibers of the present invention may be matched to themode of a single mode fiber. These include a bulk optic imaging system,as shown in FIG. 9A, a multi-mode to single-mode splice, as shown inFIG. 9B, and a tapered section of multi-mode fiber, as illustrated inFIG. 9C.

FIG. 10 is a schematic illustration of an alternative embodiment inwhich a fiber grating is used to predominantly reflect the fundamentalmode of a multi-mode fiber.

FIG. 11 is a schematic illustration of an alternative embodiment inwhich active or active-passive mode-locking is used to mode-lock themulti-mode laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates the mode-locked laser cavity 11 of this inventionwhich uses a length of multi-mode amplifying fiber 13 within the cavityto produce ultra-short, high-power optical pulses. As used herein,“ultra-short” means a pulse width below 100 ps. The fiber 13, in theexample shown, is a 1.0 m length of non-birefringent Yb³⁺/Er³⁺-dopedmulti-mode fiber. Typically, a fiber is considered multi-mode when theV-value exceeds 2.41, i.e., when modes in addition to the fundamentalmode can propagate in the optical fiber. This fiber is coiled onto adrum with a diameter of 5 cm, though bend diameters as small as 1.5 cm,or even smaller, may be used without inhibiting mode-locking. Due to theEr³⁺ doping, the fiber core in this example has an absorption ofapproximately 40 dB/m at a wavelength of 1.53 μm. The Yb³⁺ co-dopingproduces an average absorption of 4.3 dB/m inside the cladding at awavelength of 980 nm. The fiber 13 has a numerical aperture of 0.20 anda core diameter of 16 μm. The outside diameter of the cladding of thefiber 13 is 200 μm. The fiber 13 is coated with a low-index polymerproducing a numerical aperture of 0.40 for the cladding. A 10 cm lengthof single-mode Corning Leaf fiber 15 is thermally tapered to produce acore diameter of approximately 14 μm to ensure an optimum operation as amode filter, and this length is fusion spliced onto a first end 17 ofthe multi-mode fiber 13.

In this exemplary embodiment, the cavity 11 is formed between a firstminor 19 and a second minor 21. It will be recognized that other cavityconfigurations for recirculating pulses are well known, and may be used.In this example, the mirrors 19, 21 define an optical axis 23 alongwhich the cavity elements are aligned.

The cavity 11 further includes a pair of Faraday rotators 25, 27 tocompensate for linear phase drifts between the polarization eigenmodesof the fiber, thereby assuring that the cavity remains environmentallystable. As referenced herein, the phrase “environmentally stable” refersto a pulse source which is substantially immune to a loss of pulsegeneration due to environmental influences such as temperature driftsand which is, at most, only slightly sensitive to pressure variations.The use of Faraday Rotators for assuring environmental stability isexplained in more detail in U.S. Pat. No. 5,689,519 which has beenincorporated by reference herein.

A polarization beam-splitter 29 on the axis 23 of the cavity 11 ensuressingle-polarization operation of the cavity 11, and provides the output30 from the cavity. A half-wave plate 31 and a quarter-wave plate 33 areused to introduce linear phase delays within the cavity, providingpolarization control to permit optimization of polarization evolutionwithin the cavity 11 for mode-locking.

To induce mode-locking, the cavity 11 is formed as a Fabry-Perot cavityby including a saturable absorber 35 at the end of the cavity proximatethe mirror 19. The saturable absorber 35 is preferably grown as a 0.75μm thick layer of InGaAsP on one surface of a substrate. The band-edgeof the InGaAsP saturable absorber 39 is preferably chosen to be 1.56 μm,the carrier life-time is typically 5 ps and the saturation energydensity is 100 MW/cm².

In this example, the substrate supporting the saturable absorber 35comprises high-quality anti-reflection-coated InP 37, with theanti-reflection-coated surface 39 facing the open end of the cavity 11.The InP substrate is transparent to single-photon absorption of thesignal light at 1.56 μm, however two photon absorption occurs. Thistwo-photon absorber 39 is used as a nonlinear power limiter to protectthe saturable absorber 35.

The mirror 19 in this exemplary embodiment is formed by depositing agold-film onto the surface of the InGaAsP saturable absorber 35 oppositethe two photon absorber 39. The combined structure of the saturableabsorber 35, two photon absorber 37 and mirror 19 provides areflectivity of 50% at 1.56 μm. The surface of the gold-film mirror 19opposite the saturable absorber 35 is attached to a sapphire window 41for heat-sinking the combined absorber/mirror assembly.

The laser beam from the fiber 15 is collimated by a lens 43 andrefocused, after rotation by the Faraday rotator 25, by a lens 45 ontothe anti-reflection-coated surface 39 of the two-photon absorber 37. Thespot size of the laser beam on the saturable absorber 35 may be adjustedby varying the position of the lens 45 or by using lenses with differentfocal lengths. Other focusing lenses 47 and 49 in the cavity 11 aid inbetter imaging the laser signal onto the multi-mode fiber 13.

Light from a Pump light source 51, such as a laser source, with awavelength near 980 nm and output power of 5 W, is directed through afiber bundle 57 with an outside diameter of 375 μm. This pump light isinjected into the end 53 of the multi-mode fiber 13 opposite thesingle-mode fiber 17. The pump light is coupled into the cavity 11 via apump signal injector 55, such as a dichroic beam-splitter for 980/1550nm. Lenses 47 and 48 are optimized for coupling of the pump power fromthe fiber bundle 57 into the cladding of the multi-mode fiber.

The M²-value of the beam at the output 30 of this exemplary embodimentis typically approximately 1.2. Assuming the deterioration of theM²-value is mainly due to imperfect splicing between the multi-modefiber 13 and the single-mode mode-filter fiber 15, it can be estimatedthat the single-mode mode-filter fiber 15 excited the fundamental modeof the multi-mode fiber 13 with an efficiency of approximately 90%.

Mode-locking may be obtained by optimizing the focussing of the laserbeam on the saturable absorber 35 and by optimizing the orientation ofthe intra-cavity waveplates 31, 33 to permit some degree of nonlinearpolarization evolution. However, the mode-locked operation of amulti-mode fiber laser system without nonlinear polarization evolutioncan also be accomplished by minimizing the amount of mode-mixing in themulti-mode fiber 13 and by an optimization of the saturable absorber 35.

The pulses which are generated by the exemplary embodiment of FIG. 1will have a repetition rate of 66.7 MHz, with an average output power of300 mW at a wavelength of 1.535 μm, giving a pulse energy of 4.5 nJ. Atypical autocorrelation of the pulses is shown in FIG. 2. A typical FWHMpulse width of 360 fsec (assuming a sech² pulse shape) is generated. Thecorresponding pulse spectrum is shown in FIG. 3. The autocorrelationwidth is within a factor of 1.5 of the bandwidth limit as calculatedfrom the pulse spectrum, which indicates the relatively high quality ofthe pulses.

Due to the multi-mode structure of the oscillator, the pulse spectrum isstrongly modulated and therefore the autocorrelation displays asignificant amount of energy in a pulse pedestal. It can be estimatedthat the amount of energy in the pedestal is about 50%, which in turngives a pulse peak power of 6 KW, about 6 times larger than what istypically obtained with single-mode fibers at a similar pulse repetitionrate.

Neglecting the amount of self-phase modulation in one pass through themulti-mode fiber 13 and any self-phase modulation in the mode-filter 15,and assuming a linear increase of pulse power in the multi-mode fiber 13in the second pass, and assuming an effective fundamental mode area inthe multi-mode fiber 13 of 133 μm², the nonlinear phase delay in themulti-mode oscillator is calculated from the first equation above asΦ_(n1)=1.45τ, which is close to the expected maximum typical nonlineardelay of passively mode-locked lasers.

The modulation on the obtained pulse spectrum as well as the amount ofgenerated pedestal is dependent on the alignment of the minor 21.Generally, optimized mode-matching of the optical beam back into thefundamental mode of the multi-mode fiber leads to the best laserstability and a reduction in the amount of pedestal and pulse spectrummodulation. For this reason, optimized pulse quality can be obtained byimproving the splice between the single-mode filter fiber 15 and themulti-mode fiber 13. From simple overlap integrals it can be calculatedthat an optimum tapered section of Corning SMF-28 fiber 15 will lead toan excitation of the fundamental mode in the multi-mode fiber 13 with anefficiency of 99%. Thus any signal in higher-order modes can be reducedto about 1% in an optimized system.

An alternate embodiment of the invention is illustrated in FIG. 4. Asindicated by the identical elements and reference numbers, most of thecavity arrangement in this figure is identical to that shown in FIG. 1.This embodiment provides a highly integrated cavity 59 by employing aside-pumping mechanism for injecting pump light into the multi-modefiber 13. A pair of fiber couplers 61, 63, as are well known in the art,inject light from a respective pair of fiber bundles 65 and 67 into thecladding of the multi-mode fiber 13. The fiber bundles are similar tobundle 57 shown in FIG. 1, and convey light from a pair of pump sources69 and 71, respectively. Alternatively, the fiber bundles 65, 67 andcouplers 61, 63 may be replaced with V-groove light injection into themulti-mode fiber cladding in a manner well known in the art. A saturableabsorber 73 may comprise the elements 35, 37, 39 and 41 shown in FIG. 1,or may be of any other well known design, so long as it provides a highdamage threshold.

In another alternate embodiment of the invention, illustrated in FIG. 5,the laser cavity 75 includes a positive dispersion element. As with FIG.4, like reference numbers in FIG. 5 identify elements described indetail with reference to FIG. 1. In this embodiment, a section ofsingle-mode positive dispersion fiber 77 is mounted between the secondmirror 21 and the lens 49. In a similar manner, a section of positivedispersion fiber could be spliced onto the end 53 of the multi-modefiber 13, or the end of the single-mode mode-filter 15 facing the lens43. Positive dispersion fibers typically have a small core area, and maylimit the obtainable pulse energy from a laser. The embodiment shown inFIG. 5 serves to reduce the peak power injected into the positivedispersion fiber 77, and thus maximize the pulse energy output. This isaccomplished by extracting, at the polarization beam splitter 29, asmuch as 90-99% of the light energy.

In the embodiment of FIG. 5, the total dispersion inside the cavity maybe adjusted to be zero to generate high-power pulses with a largerbandwidth. Alternatively, by adjusting the total cavity dispersion to bepositive, chirped pulses with significantly increased pulse energies maybe generated by the laser.

The use of two single-mode mode-filter fibers 15, 77 is also beneficialin simplifying the alignment of the laser. Typically, to minimize modalspeckle, broad bandwidth optical signals need to be used for aligningthe mode-filter fibers with the multi-mode fiber. The use of twomode-filter fibers 15, 77 allows the use of amplified spontaneousemission signals generated directly in the multi-mode fiber for aniterative alignment of both mode-filters 15, 77.

The chirped pulses generated in the cavity 75 with overall positivedispersion may be compressed down to approximately the bandwidth limitat the frequency doubled wavelength by employing chirped periodicallypoled LiNbO₃ 79 for sum-frequency generation, in a manner well known inthe art. The chirped periodically poled LiNbO₃ 79 receives the cavityoutput from the polarization beam splitter 29 through an opticalisolator 81. In this case, due to the high power capabilities ofmulti-mode fiber oscillators, higher frequency-doubling conversionefficiencies occur compared to those experienced with single-mode fiberoscillators. Alternatively, bulk-optics dispersion compensating elementsmay be used in place of the chirped periodically poled LiNbO₃ 79 forcompressing the chirped pulses down to the bandwidth limit.

Generally, any nonlinear optical mixing technique such as frequencydoubling, Raman generation, four-wave mixing, etc. may be used in placeof the chirped periodically poled LiNbO₃ 79 to frequency convert theoutput of the multi-mode oscillator fiber 13 to a different wavelength.Moreover, the conversion efficiency of these nonlinear optical mixingprocesses is generally proportional to the light intensity or lightintensity squared. Thus, the small residual pedestal present in amulti-mode oscillator would be converted with greatly reduced efficiencycompared to the central main pulse and hence much higher quality pulsesmay be obtained.

As shown in the alternate embodiment of FIG. 6, very high-energy opticalpulses may also be obtained by inserting a chirped fiber grating such asa Bragg grating 83, with negative dispersion, into the cavity 85. Such asystem typically produces ps length, high-energy, approximatelybandwidth-limited pulses. Due to the multi-mode fiber used, much greaterpeak powers compared to single-mode fiber oscillators are generated.Here the fiber grating 83 is inserted after the polarization beamsplitter 29 to obtain an environmentally-stable cavity even in thepresence of nonpolarization maintaining multi-mode fiber 13.

In each of the embodiments of this invention, it is advantageous tominimize saturation of the multi-mode fiber amplifier 13 by amplifiedspontaneous emission generated in higher-order modes. This may beaccomplished by confining the rare-earth doping centrally within afraction of the core diameter.

Polarization-maintaining multi-mode optical fiber may be constructed byusing an elliptical fiber core or by attaching stress-producing regionsto the multi-mode fiber cladding. Examples of such fiber cross-sectionsare shown in FIGS. 7A and 7B, respectively. Polarization-maintainingmulti-mode fiber allows the construction of environmentally stablecavities in the absence of Faraday rotators. An example of such a designis shown in FIG. 8 in this case, the output of the cavity 87 is providedby using a partially-reflecting mirror 89 at one end of the cavity 87,in a manner well known in this art.

To ensure optimum matching of the fundamental mode of the multi-modefiber 13 to the mode of the single-mode mode-filter fiber 15 in each ofthe embodiments of this invention, either a bulk optic imaging system, asplice between the multi-mode fiber 13 and the single-mode fiber 15, ora tapered section of the multi-mode fiber 13 may be used. For example,the multi-mode fiber 13, either in the form shown in one for FIG. 7A andFIG. 7B or in a non-polarization maintaining form may be tapered to anoutside diameter of 70 μm. This produces an inside core diameter of 5.6μm and ensures single mode operation of the multi-mode fiber at thetapered end. By further employing an adiabatic taper, the single-mode ofthe multi-mode fiber may be excited with nearly 100% efficiency. Agraphic representation of the three discussed methods for excitation ofthe fundamental mode in an multi-mode fiber 13 with a single-mode fibermode-filter 15 is shown in FIGS. 9A, 9B and 9C, respectively. Theimplementation in a cavity design is not shown separately, but thesplice between the single-mode fiber 15 and the multi-mode fiber 15shown in any of the disclosed embodiments may be constructed with any ofthe three alternatives shown in these figures.

FIG. 10 shows an additional embodiment of the invention. Here, insteadof single-mode mode-filter fibers 15 as used in the previousembodiments, fiber gratings such as a Bragg grating directly writteninto the multi-mode fiber 13 is used to predominantly reflect thefundamental mode of the multi-mode fiber 13. Light from the pump 51 isinjected through the fiber grating 97 to facilitate a particularlysimple cavity design 99. Both chirped fiber gratings 97 as well asunchirped gratings can be implemented. Narrow bandwidth (chirped orunchirped) gratings favor the oscillation of pulses with a bandwidthsmaller than the grating bandwidth.

Finally, instead of passive mode-locking, active mode-locking oractive-passive mode-locking techniques may be used to mode-lockmulti-mode fibers. For example, an active-passive mode-locked systemcould comprise an optical frequency or amplitude modulator (as theactive mode-locking mechanism) in conjunction with nonlinearpolarization evolution (as the passive mode-locking mechanism) toproduce short optical pulses at a fixed repetition rate without asaturable absorber. A diagram of a mode-locked multi-mode fiber 13 withan optical mode-locking mechanism 101 is shown in FIG. 11. Also shown isan optical filter 103, which can be used to enhance the performance ofthe mode-locked laser 105.

Generally, the cavity designs described herein are exemplary of thepreferred embodiments of this invention. Other variations are obviousfrom the previous discussions. In particular, optical modulators,optical filters, saturable absorbers and a polarization control elementsare conveniently inserted at either cavity end. Equally, output couplingcan be extracted at an optical mirror, a polarization beam splitter oralso from an optical fiber coupler attached to the single-mode fiberfilter 15. The pump power may also be coupled into the multi-mode fiber13 from either end of the multi-mode fiber 13 or through the side of themulti-mode fiber 13 in any of the cavity configurations discussed.Equally, all the discussed cavities may be operated with any amount ofdispersion. Chirped and unchirped gratings may be implemented at eithercavity end to act as optical filters and also to modify the dispersioncharacteristics of the cavity.

What is claimed is:
 1. A laser system, comprising: a length of dopedmultimode fiber having a first end; a pump source to pump said dopedmultimode fiber, said pump source arranged for cladding pumping saiddoped multimode fiber; a first length of single mode fiber; andreflectors disposed to form a laser cavity, said cavity comprising saiddoped multimode fiber and said first single mode fiber, wherein one endof said first single mode fiber is arranged with respect to said firstend of said multimode fiber in such a way that a single mode of saidsingle mode fiber is matched to a fundamental mode of said multimodefiber, wherein an output beam of said laser system is nearly diffractionlimited and exits said multimode fiber from a second end of saidmultimode fiber opposite said first end; and a splice joining said firstend of said multimode fiber and said first single mode fiber.
 2. Thelaser system according to claim 1, further comprising a second length ofsingle mode fiber optically connected to said second end of saidmultimode fiber, said second single mode fiber disposed opposite saidfirst end of said multimode fiber and in said laser cavity.
 3. The lasersystem according to claim 1, wherein matching of the fundamental mode ofthe multi-mode fiber to the mode of the single-mode fiber is carried outwith a splice formed between the multi-mode fiber and the single-modefiber.
 4. The laser system according to claim 1, wherein matching asingle mode of said single mode fiber to a fundamental mode of saidmultimode fiber produces an excitation of the fundamental mode in themulti-mode fiber with an efficiency of 99%.
 5. The laser systemaccording to claim 1, where an M² value of said nearly diffractionlimited output beam is approximately 1.2.
 6. The laser system accordingto claim 1, wherein at least one of the reflectors comprises a fiberBragg grating or mirror.
 7. The laser system according to claim 1,wherein said multimode fiber comprises a low-index polymer producing anumerical aperture for the cladding which exceeds a numerical apertureof the multimode fiber core.
 8. The laser system according to claim 1,wherein said pump source is arranged for side pumping said dopedmultimode fiber.
 9. The laser system according to claim 1, wherein thesingle- mode fiber excites the fundamental mode of the multi-mode fiberwith an efficiency of approximately 90% to nearly 100%.